There is friction between the tire and the road which warms the tire. Warm air expands causing increase in tire pressure. That is why when you check you car's tire pressure, always do it with cold tires before you drive.

Friction between the tire and the road causes the rubber tire to have an increase in temperature, thereby transfering the heat to the air in the tire. Air expands as it gets hotter and since the air in the tire is in an enclosed environment, the pressure increases. Any moisture in the air in the tire will cause even more pressure change, and that is why race car tires are filled with heated and dryed air, and sometimes they use Nitrogen as it reacts less to heat changes.

As you ride, the walls of the tires deform where the tire flexes near the place where the tire meets the ground; called the "patch". This continuous flexing, combined with whatever heat the tires pick up from the ground itself, heats up the tires, the inner tubes, and the air inside the tubes.

Hot air expands. As it expands, it attempts to fill a greater volume. Since the volume is constrained by the rubber walls of the inner tube in the tire, the pressure increases, instead. If the air were free to fill whatever greater volume it required, the pressure would remain constant, and the volume would increase.

So, back to the bicycle. If we assume that the tire can't get bigger with the increase in pressure (it actually does stretch slightly, but let's ignore that for a moment), that means that V1 = V2; the volume of the tire is constant over time.

When Volume is held constant, and Temperature increases (T2 > T1), we see that, mathematically, Pressure must also increase by the same ratio to keep both sides of the equation equal (P2 > P1). Because V1 = V2, we can factor Volume out of the equation, which leaves us with just this:

Now, in reality, we know that the tires CAN, in fact, get bigger as the pressure goes up -- V2 is a little bit more than V1 -- because they're made of a flexible material: rubber. This means that, to the extent that V2 is greater than V1, the change in Temperature from T1 to T2 does NOT have to be 100% compensated for by a change in Pressure from P1 to P2. In other words, SOME of the change in Temperature is compensated by an increase in Volume and the rest by an increase in Pressure.

Though it is more simple to illustrate how the equation works by keeping one factor constant -- effectively eliminating it from the equation -- in the real world, all three factors can change.

Friction between the tire and the road causes the rubber tire to have an increase in temperature, thereby transfering the heat to the air in the tire. Air expands as it gets hotter and since the air in the tire is in an enclosed environment, the pressure increases. Any moisture in the air in the tire will cause even more pressure change, and that is why race car tires are filled with heated and dryed air, and sometimes they use Nitrogen as it reacts less to heat changes.As you ride, the walls of the tires deform where the tire flexes near the place where the tire meets the ground; called the "patch". This continuous flexing, combined with whatever heat the tires pick up from the ground itself, heats up the tires, the inner tubes, and the air inside the tubes More Questions & Answers ...